1 / 62

Assemblers

Assemblers. System Software by Leland L. Beck Chapter 2. Role of Assembler. Assembler. Object Code. Linker. Source Program. Executable Code. Loader. Chapter 2 -- Outline. Basic Assembler Functions Machine-dependent Assembler Features Machine-independent Assembler Features

aida
Download Presentation

Assemblers

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Assemblers System Software by Leland L. Beck Chapter 2

  2. Role of Assembler Assembler Object Code Linker Source Program Executable Code Loader

  3. Chapter 2 -- Outline • Basic Assembler Functions • Machine-dependent Assembler Features • Machine-independent Assembler Features • Assembler Design Options

  4. Introduction to Assemblers • Fundamental functions • translating mnemonic operation codes to their machine language equivalents • assigning machine addresses to symbolic labels • Machine dependency • different machine instruction formats and codes

  5. Example Program (Fig. 2.1) • Purpose • reads records from input device (code F1) • copies them to output device (code 05) • at the end of the file, writes EOF on the output device, then RSUB to the operating system

  6. Example Program (Fig. 2.1) • Data transfer (RD, WD) • a buffer is used to store record • buffering is necessary for different I/O rates • the end of each record is marked with a null character (0016) • the end of the file is indicated by a zero-length record • Subroutines (JSUB, RSUB) • RDREC, WRREC • save link register first before nested jump

  7. Assembler Directives • Pseudo-Instructions • Not translated into machine instructions • Providing information to the assembler • Basic assembler directives • START • END • BYTE • WORD • RESB • RESW

  8. Assembler’s functions • Convert mnemonic operation codes to their machine language equivalents • Convert symbolic operands to their equivalent machine addresses  • Build the machine instructions in the proper format • Convert the data constants to internal machine representations • Write the object program and the assembly listing

  9. Example of Instruction Assemble • Forward reference STCH BUFFER,X 549039 (54)16 1 (001)2 (039)16

  10. Difficulties: Forward Reference • Forward reference: reference to a label that is defined later in the program. LocLabelOperatorOperand 1000 FIRST STL RETADR 1003 CLOOP JSUB RDREC … … … … … 1012 J CLOOP … … … … … 1033 RETADR RESW 1

  11. Two Pass Assembler • Pass 1 • Assign addresses to all statements in the program • Save the values assigned to all labels for use in Pass 2 • Perform some processing of assembler directives • Pass 2 • Assemble instructions • Generate data values defined by BYTE, WORD • Perform processing of assembler directives not done in Pass 1 • Write the object program and the assembly listing

  12. Intermediate file Two Pass Assembler • Read from input line • LABEL, OPCODE, OPERAND Source program Object codes Pass 1 Pass 2 OPTAB SYMTAB SYMTAB

  13. Data Structures • Operation Code Table (OPTAB) • Symbol Table (SYMTAB) • Location Counter(LOCCTR)

  14. OPTAB (operation code table) • Content • menmonic, machine code (instruction format, length) etc. • Characteristic • static table • Implementation • array or hash table, easy for search

  15. SYMTAB (symbol table) • Content • label name, value, flag, (type, length) etc. • Characteristic • dynamic table (insert, delete, search) • Implementation • hash table, non-random keys, hashing function COPY 1000 FIRST 1000 CLOOP 1003 ENDFIL 1015 EOF 1024 THREE 102D ZERO 1030 RETADR 1033 LENGTH 1036 BUFFER 1039 RDREC 2039

  16. Object Program • Header Col. 1 H Col. 2~7 Program name Col. 8~13 Starting address (hex) Col. 14-19 Length of object program in bytes (hex) • Text Col.1 T Col.2~7 Starting address in this record (hex) Col. 8~9 Length of object code in this record in bytes (hex) Col. 10~69 Object code (69-10+1)/6=10 instructions • End Col.1 E Col.2~7 Address of first executable instruction (hex) (END program_name)

  17. Fig. 2.3 H COPY 001000 00107A T 001000 1E 141033 482039 001036 281030 301015 482061 ... T 00101E 15 0C1036 482061 081044 4C0000 454F46 000003 000000 T 002039 1E 041030 001030 E0205D 30203F D8205D 281030 … T 002057 1C 101036 4C0000 F1 001000 041030 E02079 302064 … T 002073 07 382064 4C0000 05 E 001000

  18. Homework #1 SUM START 4000 FIRST LDX ZERO LDA ZERO LOOP ADD TABLE,X TIX COUNT JLT LOOP STA TOTAL RSUB TABLE RESW 2000 COUNT RESW 1 ZERO WORD 0 TOTAL RESW 1 END FIRST End of Sec 2-1

  19. Assembler Design • Machine Dependent Assembler Features • instruction formats and addressing modes • program relocation • Machine Independent Assembler Features • literals • symbol-defining statements • expressions • program blocks • control sections and program linking

  20. Machine-dependent Assembler Features Sec. 2-2 Instruction formats and addressing modes Program relocation

  21. Instruction Format and Addressing Mode • SIC/XE • PC-relative or Base-relative addressing: op m • Indirect addressing: op @m • Immediate addressing: op #c • Extended format: +op m • Index addressing: op m,x • register-to-register instructions • larger memory -> multi-programming (program allocation) • Example program • Figure 2.5

  22. Translation • Register translation • register name (A, X, L, B, S, T, F, PC, SW) and their values (0,1, 2, 3, 4, 5, 6, 8, 9) • preloaded in SYMTAB • Address translation • Most register-memory instructions use program counter relative or base relative addressing • Format 3: 12-bit address field • base-relative: 0~4095 • pc-relative: -2048~2047 • Format 4: 20-bit address field

  23. PC-Relative Addressing Modes • PC-relative • 10 0000 FIRST STL RETADR 17202D (14)16 1 1 0 0 1 0 (02D) 16 • displacement= RETADR - PC = 30-3 = 2D • 40 0017 J CLOOP 3F2FEC (3C)16 1 1 0 0 1 0 (FEC) 16 • displacement= CLOOP-PC= 6 - 1A= -14= FEC

  24. Base-Relative Addressing Modes • Base-relative • base register is under the control of the programmer • 12 LDB #LENGTH • 13 BASE LENGTH • 160 104E STCH BUFFER, X 57C003 ( 54 )16 1 1 1 1 0 0 ( 003 ) 16 (54) 1 1 1 0 1 0 0036-1051= -101B16 • displacement= BUFFER - B = 0036 - 0033 = 3 • NOBASE is used to inform the assembler that the contents of the base register no longer be relied upon for addressing

  25. Immediate Address Translation • Immediate addressing • 55 0020 LDA #3 010003 ( 00 )16 0 1 0 0 0 0 ( 003 ) 16 • 133 103C +LDT #4096 75101000 ( 74 )16 0 1 0 0 0 1 ( 01000 ) 16

  26. Immediate Address Translation(Cont.) • Immediate addressing • 12 0003 LDB #LENGTH 69202D ( 68)16 0 1 0 0 1 0 ( 02D ) 16 ( 68)16 0 1 0 0 0 0 ( 033)16 690033 • the immediate operand is the symbol LENGTH • the address of this symbol LENGTH is loaded into register B • LENGTH=0033=PC+displacement=0006+02D • if immediate mode is specified, the target address becomes the operand

  27. Indirect Address Translation • Indirect addressing • target addressing is computed as usual (PC-relative or BASE-relative) • only the n bit is set to 1 • 70 002A J @RETADR 3E2003 ( 3C )16 1 0 0 0 1 0 ( 003 ) 16 • TA=RETADR=0030 • TA=(PC)+disp=002D+0003

  28. Program Relocation • Example Fig. 2.1 • Absolute program, starting address 1000 e.g. 55 101B LDA THREE 00102D • Relocate the program to 2000 e.g. 55 101B LDA THREE 00202D • Each Absolute address should be modified • Example Fig. 2.5: • Except for absolute address, the rest of the instructions need not be modified • not a memory address (immediate addressing) • PC-relative, Base-relative • The only parts of the program that require modification at load time are those that specify direct addresses

  29. Example

  30. Relocatable Program • Modification record • Col 1 M • Col 2-7 Starting location of the address field to be modified, relative to the beginning of the program • Col 8-9 length of the address field to be modified, in half- bytes

  31. Object Code End of Sec 2-2

  32. Machine-Independent Assembler Features Literals Symbol Defining Statement Expressions Program Blocks Control Sections and Program Linking

  33. Literals • Design idea • Let programmers to be able to write the value of a constant operand as a part of the instruction that uses it. • This avoids having to define the constant elsewhere in the program and make up a label for it. • Example • e.g. 45 001A ENDFIL LDA =C’EOF’ 032010 • 93 LTORG • 002D * =C’EOF’ 454F46 • e.g. 215 1062 WLOOP TD =X’05’ E32011

  34. Literals vs. Immediate Operands • Immediate Operands • The operand value is assembled as part of the machine instruction • e.g. 55 0020 LDA #3 010003 • Literals • The assembler generates the specified value as a constant at some other memory location • e.g. 45 001A ENDFIL LDA =C’EOF’ 032010 • Compare (Fig. 2.6) • e.g. 45 001A ENDFIL LDA EOF 032010 • 80 002D EOF BYTE C’EOF’ 454F46

  35. Literal - Implementation (1/3) • Literal pools • Normally literals are placed into a pool at the end of the program • see Fig. 2.10 (END statement) • In some cases, it is desirable to place literals into a pool at some other location in the object program • assembler directive LTORG • reason: keep the literal operand close to the instruction

  36. Literal - Implementation (2/3) • Duplicate literals • e.g. 215 1062 WLOOP TD =X’05’ • e.g. 230 106B WD =X’05’ • The assemblers should recognize duplicate literals and store only one copy of the specified data value • Comparison of the defining expression • Same literal name with different value, e.g. LOCCTR=* • Comparison of the generated data value • The benefits of using generate data value are usually not great enough to justify the additional complexity in the assembler

  37. Literal - Implementation (3/3) • LITTAB • literal name, the operand value and length, the address assigned to the operand • Pass 1 • build LITTAB with literal name, operand value and length, leaving the address unassigned • when LTORG statement is encountered, assign an address to each literal not yet assigned an address • Pass 2 • search LITTAB for each literal operand encountered • generate data values using BYTE or WORD statements • generate modification record for literals that represent an address in the program

  38. Symbol-Defining Statements • Labels on instructions or data areas • the value of such a label is the address assigned to the statement • Defining symbols • symbol EQU value • value can be:  constant,  other symbol,  expression • making the source program easier to understand • no forward reference

  39. Symbol-Defining Statements • Example 1 • MAXLEN EQU 4096 • +LDT #MAXLEN • Example 2 • BASE EQU R1 • COUNT EQU R2 • INDEX EQU R3 • Example 3 • MAXLEN EQU BUFEND-BUFFER • +LDT #4096

  40. ORG (origin) • Indirectly assign values to symbols • Reset the location counter to the specified value • ORG value • Value can be:  constant,  other symbol,  expression • No forward reference • Example • SYMBOL: 6bytes • VALUE: 1word • FLAGS: 2bytes • LDA VALUE, X

  41. ORG Example • Using EQU statements • STAB RESB 1100 • SYMBOL EQU STAB • VALUE EQU STAB+6 • FLAG EQU STAB+9 • Using ORG statements • STAB RESB 1100 • ORG STAB • SYMBOL RESB 6 • VALUE RESW 1 • FLAGS RESB 2 • ORG STAB+1100

  42. Expressions • Expressions can be classified as absolute expressions or relative expressions • MAXLEN EQU BUFEND-BUFFER • BUFEND and BUFFER both are relative terms, representing addresses within the program • However the expression BUFEND-BUFFER represents an absolute value • When relative terms are paired with opposite signs, the dependency on the program starting address is canceled out; the result is an absolute value

  43. SYMTAB • None of the relative terms may enter into a multiplication or division operation • Errors: • BUFEND+BUFFER • 100-BUFFER • 3*BUFFER • The type of an expression • keep track of the types of all symbols defined in the program

  44. Example 2.9 SYMTAB LITTAB

  45. Program Blocks • Program blocks • refer to segments of code that are rearranged within a single object program unit • USE [blockname] • At the beginning, statements are assumed to be part of the unnamed (default) block • If no USE statements are included, the entire program belongs to this single block • Example: Figure 2.11 • Each program block may actually contain several separate segments of the source program

  46. Program Blocks - Implementation • Pass 1 • each program block has a separate location counter • each label is assigned an address that is relative to the start of the block that contains it • at the end of Pass 1, the latest value of the location counter for each block indicates the length of that block • the assembler can then assign to each block a starting address in the object program • Pass 2 • The address of each symbol can be computed by adding the assigned block starting address and the relative address of the symbol to that block

  47. Figure 2.12 • Each source line is given a relative address assigned and a block number • For absolute symbol, there is no block number • line 107 • Example • 20 0006 0 LDA LENGTH 032060 • LENGTH=(Block 1)+0003= 0066+0003= 0069 • LOCCTR=(Block 0)+0009= 0009

  48. Program Readability • Program readability • No extended format instructions on lines 15, 35, 65 • No needs for base relative addressing (line 13, 14) • LTORG is used to make sure the literals are placed ahead of any large data areas (line 253) • Object code • It is not necessary to physically rearrange the generated code in the object program • see Fig. 2.13, Fig. 2.14

  49. Control Sectionsand Program Linking • Control Sections • are most often used for subroutines or other logical subdivisions of a program • the programmer can assemble, load, and manipulate each of these control sections separately • instruction in one control section may need to refer to instructions or data located in another section • because of this, there should be some means for linking control sections together • Fig. 2.15, 2.16

  50. External Definition and References • External definition • EXTDEF name [, name] • EXTDEF names symbols that are defined in this control section and may be used by other sections • External reference • EXTREF name [,name] • EXTREF names symbols that are used in this control section and are defined elsewhere • Example • 15 0003 CLOOP +JSUB RDREC 4B100000 • 160 0017 +STCH BUFFER,X 57900000 • 190 0028 MAXLEN WORD BUFEND-BUFFER 000000

More Related